Polymer swelling by scCO2 was mainly studied to understand many industrial processes, especially in the field of pharmacy for drug delivery (impregnation) and also polymer processing by CO2-assisted extrusion. We have studied here another application of polymer swelling by scCO2 for the synthesis of nanocomposites. The selected model system was the direct synthesis of copper nanoparticles in a poly(ethylene glycol) matrix (PEG). The study of the formation of nanostructures in polymer matrix is constituted of three main steps: i) thermodynamical behaviour of the polymer/CO2 system, ii) viscosity of the polymer/CO2 system and iii) nucleation and growth of copper nanoparticles. First, the thermodynamical behaviour of the PEG/CO2 system was studied by in situ IR spectroscopy. This method gives accurate values of the polymer swelling by scCO2 but also of the CO2 sorption in the polymer. For example, at 40°C-15MPa, the swelling of PEG is equal to 35% for a CO2 solubility of 23 wt%. Secondly, we have developed an original falling ball viscometer for the determination of PEG viscosity as a function of CO2 density. A good knowledge of the polymer/CO2 system is finally used for the control of formation of copper nanoparticles. This formation was followed by in situ UV - visible spectroscopy and characterized by transmission electron microscopy.